The acid-catalyzed hydrolysis of an ester and the inversion of cane sugar are examples of pseudo-unimolecular reactions.
1) Acid-catalyzed hydrolysis of an ester:
In this reaction, an ester molecule reacts with water in the presence of an acid catalyst to form an alcohol and a carboxylic acid. The general reaction equation can be represented as:
\(R-COOR'\text{ + }H_2O\text{ + }H^+ −−−→ R-COOH\text{ + }R'-OH\)
Here, R and R' represent organic groups.
At first glance, this reaction may appear to proceed through a unimolecular mechanism because it involves the ester molecule undergoing hydrolysis. However, a closer examination reveals that water molecules are also involved in the reaction. The acid catalyst donates a proton (H+) to the carbonyl oxygen of the ester, making it more susceptible to nucleophilic attack by water. The water molecule then attacks the carbonyl carbon, leading to the formation of an intermediate alkoxide ion. This intermediate subsequently protonates to form the alcohol and carboxylic acid.
Since the reaction involves both the ester molecule and the water molecule, it can be considered a bimolecular reaction. Therefore, it is not a true unimolecular reaction but a pseudo unimolecular reaction.
2) Inversion of cane sugar:
The inversion of cane sugar refers to the hydrolysis of sucrose (a disaccharide) into its constituent monosaccharides (glucose and fructose). The reaction can be catalyzed by either acid or enzymes.
\(Sucrose\text{ + }H_2O\text{ + }Acid\text{ or }Enzyme −−−→ Glucose\text{ + }Fructose\)
The reaction proceeds through the cleavage of the glycosidic bond between glucose and fructose. Initially, the acid or enzyme catalyst donates a proton to the glycosidic oxygen, breaking the bond and forming a carbocation intermediate. Water molecules then attack the carbocation from either side, leading to the inversion of the configuration of the anomeric carbon and the formation of glucose and fructose.
Similar to the acid-catalyzed hydrolysis of an ester, this reaction involves both the sucrose molecule and water molecules. The acid or enzyme catalyst aids in the reaction by facilitating the protonation step. Therefore, the inversion of cane sugar can be considered a pseudo-unimolecular reaction.
In summary, both the acid-catalyzed hydrolysis of an ester and the inversion of cane sugar are examples of pseudo-unimolecular reactions because they involve multiple molecules (ester + water or sucrose + water) despite initially appearing to proceed through a unimolecular mechanism. |
